I decided to take a look at the 3DS game Yo-kai Watch today but unfortuantely after decrypting and unpacking the ROM, it appears all data except for audio and pre-rendered video is packed into one single huge archive (~344MB). File extension is .fa, but the header magic appears to be ARC0H. I uploaded two 10MB examples from the beginning and end here, please have a look: https://mega.co.nz/#!tcozzRzQ!SjM_RzXxI ... Drvm9YP3z0
I can easily find a lot of Japanese text like the game title, menu options etc in the file, so I guess there are plain text sections. Would be great to have an unpacker and repacker, if somebody feels like doing it

E: There is an unpacker on github https://github.com/polaris-/3ds-xfsatool
Cheers!